Development of a perforated plate underwater acoustic ground cloak.

One of the commonly investigated transformation acoustic device is the ground cloak, which conceals a scattering object on a reflecting surface. Multiple studies have numerically simulated acoustic ground cloaks, but because of the challenges associated with realizing a homogeneous anisotropic metamaterial, only two acoustic ground cloaks have been built and tested. Perforated plastic plates in air were used to construct two and three dimensional ground cloaks and alternating layers of brass and water were used to construct an extended area ground cloak underwater. With underwater mass density anisotropy previously demonstrated for perforated steel plates, the primary focus of this article is to build and evaluate an underwater ground cloak with perforated steel plates. The cloak was evaluated at a water-air pressure release reflecting surface. The cloak successfully concealed the scattering object over a broad frequency range of 7-12 kHz.